352 CHEMICAL ENGINEERING
or to become stratified in the solution. If they do settle out in this interval there is no positive means of raising them and that brings up the last objection to this type and that is that it has been found in practice that there is not an even distribution of solids through the volume of a paddle agitator there being a much greater concentration in the lower layers than in the top.
Propeller Type.—This type consists merely of a propeller revolved in the bottom of a tank so as to throw the solution and solids up to the top of the tank where they are then sucked down by the propeller and shot up again. Baffles are placed along the side of the tank to counteract the slight tendency of the solution to rotate with the propeller. The criticism of this type is the same as with the baffle-paddle type. We start off as though what we actually wanted to do was merely to set up a lot of eddy currents of solid and liquid, whereas what we really want to do is to set up a selective current of the particles in the solution. The advantage it has over the baffle-paddle type is that there is a positive means of raising any particles tending to settle out and keeping them in motion and the distribution should be very good.
A combination of the propeller and paddle-agitator is got by inclining the bottom set of paddles so as to keep an upward motion of the solution in the tank.
Air Agitation.—This is the simplest of all agitators in many ways, the cheapest and simplest to install and the most easy way to waste air. It consists of a perforated pipe placed in the bottom of the tank and connected to a compressed air line. It gives a violent agitation and prevents very effectively settling from taking place. The efficient use of the air requires that it be evenly discharged over the entire area of the tank and in as small bubbles as possible. This is impossible to do and in most cases the bubbles are very large and are restricted over a small portion of the tank due to the holes having become clogged during the intervals when the machine was not in operation. Here again the same objection is raised that there is no attempt made to take a positive action to guarantee a relative motion between the undissolved solids and the solution. We practically try to raise the entire tank full of materials all the time, whereas in a latter paragraph we will mention other types of agitators which use compressed air to raise selectively the solids and naturally their power consumption is a very small fraction of that used by this type.
Dorr Agitator.—This is an ordinary tank in the center of which is a vertical shaft to the bottom of which are attached two arms with rakes, so that when the shaft is revolved they will rake any solids which settle out towards the center. This shaft in the center is hollow and has openings at the bottom and top and it is made into an air lift by a compressed-air pipe leading in at the bottom. To the top of the shaft are attached two perforated launders which revolve with the shaft and any material coming up through the center of the air lift flows into them and will be distributed over the entire surface of the solution in the tank.
If we place in a Dorr agitator a solid suspended in water the solids will of course tend to sink to the bottom as the arms revolve at such a low rate that there is practically no agitation by them. In sinking they pass through the solution and thus the film of concentrated solution is continually washed away. When they reach the bottom the rakes take them to the center of the tank and deliver them to the air lift which shoots them up to the launders where they are distributed over the surface of the solution and allowed to percolate through the liquid or solution again. Here